The present invention relates to an optical system of variable focal and back-focal length. It is intended primarily for use in operation microscopes, but it can also be used in other optical systems, for instance in stereo microscopes in industry (technical microscopes), or in monocular viewing instruments.
In the use of stereo microscopes in medicine and in industry, it is necessary to be able to adapt the working distance to different heights of the object without changing the position of the heavy microscope body, and to be able to change the focal length of the stereo microscope with a fixed working distance.
From West German Offenlegungsschrift (published patent application) No. 24 39 820 published on Mar. 4, 1976, it is known, for instance, to provide an operation microscope with a focusable main objective in order to adapt the working distance to different heights of object without changing the position of the microscope body. This known solution has the disadvantage that it requires a large structural height of the microscope body. For a large range of back-focuses a large path of displacement of the objective is also necessary. A change in the focal length with fixed working distance is not possible in this known operation microscope.
The object of the present invention, therefore, is to provide an optical system which can satisfy effectively both the need of adapting the working distance to different heights of object and of changing the focal length with a fixed working distance.